Planet interiors Flashcards
relative moment of inertia about the x,y,z axis of a planet
z: the moment of inertia about the axis through the poles
Z > Y (if the planet has an equatorial bulge)
Y=X (if symmetric)
how can the moments of inertia be measured for a planet
how:
deviations in a satellite’s orbit are caused by the equatorial bulge. these are used to measure the gravitational field. knowing this, we can determine the difference in the MoI (Z - X)
how can the MoI about the z-axis be determined from (Z-X)
- measure the spin precession rate about the planet’s spin axis. This depends on (Z-X)/Z
- If the planet’s a fluid, its flattening comes from the angular velocity and MoI about the z-axis
what are P and S waves
P-waves: longitudinal, primary waves
S-waves: transverse, secondary waves
P and S wave shadow zones and what causes them
P-waves: 104 - 140 degrees from the point of origin
S-waves: 104- 180 degrees from the point of origin
(that’s in both clockwise and anti-clockwise directions)
S-waves:
can’t travel through the liquid outer-core
P-waves:
Refracted as its velocity increases with depth. Also refracted at the mantle-core interface
mechanisms that heated the interior
residual heat: impacts falling to the centre liberate G.P.E
gravitational settling: Heavier elements sinking to the centre, liberating G.P.E
internal radioactive decay: significant for rocky planets
tidal forces: most significant in satellites
rate of planetary cooling
internal heat is proportional to the volume: ∝ R³
cooling is proportional to the surface area: ∝ R²
hence the timescale of cooling is proportional to R.
therefore larger planets take longer to cool
what planets don’t have a B-field and why
venus and mars
Mars:
it has a residual field at the surface, indicating it used to have one.
Unknown why it doesn’t anymore. (due to large impactor?)
Venus:
Surface is too hot to have residual B-fields. Its rotation is probably too slow to generate the currents required in the core.
